I can't give you an opinion on comparison as I've never used Amazon myself, however, your question does raise a bit of a question for me.

You say that you already have a supplier account, yet you're unsure about your actual marketplace. I'm just wondering if you have done your market research on both platforms and discovered that this supplier is the perfect fit for you or are you putting the cart before the horse here, opening an account with a supplier before knowing what the competition is like on those two platforms within your particular market genre?

Research is certainly king in e-commerce, the more you research, the more you increase your chances of success. Unfortunately I have no clue as to any current niche markets and if I did, then I would probably be taking advantage of that knowledge myself lol, but that's where research comes into it.

I tend to think that the word "niche" gets throw around a little too casually these days. True niche markets are very difficult to find and most people end up stumbling upon them just through test marketing products and that for me is still the best way to confirm any market. That's the beauty of dropshipping, it provides you with a cost effective avenue to test market products and develop markets.

You really don't need to find a "niche" market as such, you just need to find markets that aren't saturated and have a steady, even if slow, sell through rate. Put enough of those type of markets in your back pocket and you can start building a successful e-commerce business.

It is of course just my own opinion, but if you aren't using a wholesale business model and a very good one, then you're far better just staying clear of saturated high turn over markets because the competition is just too heavy. There are some exceptions to that of course, one of those is looking at bundling options to break into that type of market for example, but going product to product, dropship is never going to cut it against a wholesale seller.

This is a common question for start-up sellers. There is no right or wrong answers though as the answer depends on your target market and the type of products that you would be offering.

Just to give you an idea, I can give you salient points to compare both sites. Online traffic vote would go for Amazon while eBay has less browser issues and is mostly compatible with all browsers. The demographics of Amazon users are younger than those of eBay users. As for fees, Amazon generally charge more at around 15% while eBay only takes around 10%. eBay is easier to navigate for first time sellers vs Amazon. And one of the most important things to consider into is: eBay has higher selling likelihood for less popular or older items while Amazon are mostly for brand-new and/or in demand items.

While there is the unending comparison between Amazon and eBay, do keep in mind that there are also other e-Commerce sites that you can venture into such as Link hidden: Login to view. Having your own online store means no direct competition and unlimited growth potential.